Metal coating composition and process



United States Patent 3,404,043 METAL COATING COMPOSITION AND PROCESSGaillard W. Dell, Detroit, Mich., assignor to Hooker ChemicalCorporation, Niagara Falls, N.Y., a corporation of New York No Drawing.Filed Nov. 10, 1964, Ser. No. 410,296 16 Claims. (Cl. 148-6.2)

ABSTRACT OF THE DISCLOSURE- A composition for forming a protectivecoating on aluminum-containing surfaces which consists essentially of anaqueous acidic solution containing hexavalent chromium ions, fluorideions, and at least 0.01% molybdenum as an activator for the composition.The aluminum surface to be coated is maintained in contact with thissolution for a period sufficient to form the desired coating thereon.

This invention relates to an improved composition and process forcoating metal surfaces and more particularly relates to improvedcompositions and methods for chemically coating aluminum and aluminumalloy surfaces to provide corrosion resistant and paint bonding coatingon such surfaces.

Many compositions and processes has been proposed in the past forforming chemical films on surfaces of 'aluminum, and particularlysurfaces of aluminum and aluminum alloys. From a commercial standpoint,the most successful of these processes are those wherein the aluminumcontaining surface to be coated is contacted with an acidic solutioncontaining a source of hexavalent chromium. In such processes, theattempt has been made to provide a method which is not only useful intreating various types of aluminum and aluminum alloys, but also toprovide a coating solution which is relatively simple to prepareinitially and which is easily maintained in effective coating conditionduring use.

For the most part, these prior art compositions have been aqueous acidicsolutions containing a source of hexavalent chromium ions and a sourceof fluoride ions. It has also been the general practice to add to thesebasic compositions various additional anions or cations in order toobtain specific objectives, such as activation of the solution in orderto obtain increased coating weight and coating efficiency. Exemplary ofsuch modified compositions is that set forth in U.S. Patent 2,796,370,wherein ferricyanide is utilized as the modifying or activating anion.

In the commercial operation of chromate-fluoride, ferricyanide activatedsolutions for coating aluminum, a number of operational diflicultieshave been encountered. These solutions must always contain a certainminimum quantity of ferricyanide ion in order to preserve the coatingrate and coating efficiency in the solution at levels which arecommercially acceptable from the standpoints of both quality andeconomy. It is known that the ferricyanide ion is sensitive totemperature degradation at temperatures above about 50 degrees C., andunless care is constantly exercised to avoid exceeding this temperature,even at localized areas within the operating tank, the ferricyanidebecomes degraded and incapable of maintaining the solution in acceptablecoating-forming condition. Additionally, it has been found that theseferricyanide activated solutions are relatively sensitive to acidityand, thus, require considerable maintenance of the necessary aciditywithin a fairly narrow range. Moreover, these solutions are found tolose their coating ability merely upon standing at room temperaturewithout use and the replenishment of them is complicated because "icethe ferricyanide and chromate-fluoride component must be addedseparately to the solution and must be maintained, prior to addition, inseparate containers in order to avoid rea-ctionbetween these components.

It is, therefore, an object of the present invention to provide animproved chromate-fluoride coating solution which is capable ofoperations at high speeds and high coating efliciencies to form acorrosion resistant, paint receptive coating on aluminum containingsurfaces.

Another obect of the present invention is to provide an improvedchromate-fiuoride coating solution which is capable of operating at highcoating rates and high coating efficiencies over a wide range of pH andtemperature values.

A further object of the present invention is to provide an improvedprocess for coating aluminum containing surfaces, which process iscapable of being maintained in optimum coating-forming condition byreplenishing with a single replenishing material.

A still further object of the invention is to provide an improvedprocess for coating aluminum surfaces which process is economical touse, easy to control and maintain in optimum coating-forming conditionand which is capable of providing adherent, corrosion resistantcoatings.

These and other objects of the invention will become apparent to thoseskilled in the art from the description of the invention which follows.

Pursuant to the above objects, the present invention includes a coatingcomposition which comprises an aqueous acidic solution, substantiallyfree of lithium ions, antimony ions, tin ions and cations of Group II-Aof the periodic table and which contains hexavalent chromium ions,fluoride ions and at least 0.01 percent of molybdenum. 5

More specifically, the composition of the present invention is anaqueous acidic solution which is substantially free of lithium ions,antimony ions, tin ions and cations of Group II-A of the periodic table,namely beryllium, magnesium, calcium, strontium, and barium. It has beenfound, in the present composition, that not only do the above ions notadd any beneficial properties to the composition but, in many instances,are in fact detrimental to the operation and maintenance of the coatingsolution and to the protective coatings which are produced. Accordingly,it is desirable that the coating solutions of the present invention besubstantially free of these ions. By substantially free, it is meantthat the solutions are free of at least amounts of these ions which aredetrimental to the solution or coating produced. It is not necessarilyintended, however, to exclude minor amounts of these ions which are notdetrimental, such as amounts which may occur in the water used in makingup the aqueous solution, e.g., less than about 0.05% by weight of thesolution.

The aqueous acidic coating solutions of the present invention containhexavalent chromium ions in an amount suflicient to provide the desiredchromium coating on the aluminum surfaces treated therewith. Desirably,these solutions contain hexavalent chromium ions, calculated as CrO inan amount within the range of about 0.05 to about 1 percent,weight/volume (w./v.), i.e., percent weight per unit volume of solution.The hexavalent chromium ions may be added to the solution in manysuitable forms, such as chromic acid, or one or more of the watersoluble or water dispersable salts of chromic acid. Exemplary of thesalts which may be used are the sodium, potassium or ammonium salts suchas the chromates and dichromates, as well as admixtures thereof bothwith each other and/or with chromic acid.

The fluoride ion is present in the subject composition in an amountsufficient to cause attack of the aluminum containing surface to becoated and to effect formation of the resulting coating on this surface.Desirably, the fluoride ion is present in an amount within the range ofabout 0.16 to about 2.7 percent w./v. As with the hexavalent chromiumion, the fluoride ions may be added to the present composition in manyconvenient forms, including various fluorine-containing compounds whichare capable of ionizing in the aqueous acidic solutions of the inventionto provide fluoride ions. Exemplary of such fluorine containingcompounds which may be used are hydrofluoric acid, fluosilicic acid,fluoboric acid, as well as the various water soluble or waterdispersable salts thereof such as the sodium, potassium, and ammoniumsalts.

As has been indicated thereinabove, there is also included in thepresent composition, at least 0.01 percent w./v. of molybdenum. It isbelieved that in the present composition, the molybdenum acts as anactivator which makes it possible to obtain and maintain high coatingweights and coating efliciencies. Although the maximum amount of themolybdenum used has not been found to be critical, it has been foundthat when amounts of molybenum substantially in excess of about 0.4percent w./v. are used, no appreciable additional improvements incoating weight and coating efficiencies are obtained. Accordingly, it isdesirable that the molybdenum be present in the coating solution inamounts within the range of about 0.01 to about 0.4 percent w./v. Themolybdenum may be added to the coating solution in the form of variouscompounds which are ionizable in the solution, such as molybdic acid, aswell as various water soluble or water dispersable salts thereof whichwill provide the desired molybdenum ion when oxidized in the solution bythe chromic acid or chromic acid salts. Exemplary of such molybdenumcontaining salts which may be used are the sodium, potassium, orammonium salts of molybdic acid.

In addition to the above components, in many instances it has also beenfound desirable to include in the present composition an aluminumfluoride complex ion, which ion is desirably present in an amountequivalent to about 0.22 to about 3.2 percent w./v. of Al(F) Thealuminum fluoride complex ion is expressed as Al(F) because, when used,it is present in the operating solution as an equilibrium of Al(F) ionswhich may contain from 1 to 6 fluoride atoms per aluminum atom. In thesolutions of the present invention, it has been found that thisequilibrium averages out to be approximately equivalent to the Al(F)ion. Accordingly, as used in the specification and claims, theexpression Al(F) is intended to represent any aluminum fluoride ion andthe quantities thereof refer to an amount of such ion equivalent to theAl(F) ion. This aluminum fluoride complex ion, when used, may beincorporated in the present coating solution as such, or may be formedin the solution as a complex from free aluminum and fluoride ions. Inthe latter instance, the fluoride may be present as hydrofluoric acid,fluoboric fluosilicic acid, or the like. Where the aluminum fluoridecomplex is added as such, it may be prepared by dissolving aluminumoxide (Al O in water and hydrofluoric acid in appropriate proportions toobtain the requisite parts of Al(F) for the composition.

Particularly preferred compositions of the present invention are thosefalling Within the following formulation:

Concentration Solution component: percent w./v.

Hexavalent chromium ions (calculated as CrO 0.2 0.5 Fluoride ions0.l6-1.6 Molybdenum 0.03-0.1 Al(F) complex 0.221.9

The solutions of the Present invention may be used to form coatings onsurfaces containing aluminum, such as aluminum itself or aluminum alloyswhich are predominantly aluminum, using various coating techniques, suchas dipping, brushing, spraying, flooding, or the like. Preferably, thesolutions of the present invention are applied to the aluminum surfacesafter these surfaces have been subjected to conventional cleaningprocedures which free the surface to be treated of oil, grease, oxides,and the like. Additionally, the present solutions may be applied to thealuminum surfaces by atomizing the solution on the surface in a heatedcondition. Generally stated, this atomization application techniqueincludes the steps of preliminarily heating the aluminum or aluminumalloy surface to be coated to a temperature above about degreescentigrade and atomizing on the heated surface a quantity of the coatingsolution suflicient to form the desired coating but insufficient tocause the droplets of atomized coating solution to coalesce or puddle onthe surface. The coating on the aluminum surface results from thesubstantially instantaneous flashing or volatilization of the liquidfrom the solution, so that each individual atomized particle dropletremains substantially in the locus of its original contact with thesurface treated.

Although the coating process of the present invention may be carried outeffectively and with good efliciency over a wide range of solutiontemperatures and solution acidities, it has been found that the rate ofcoating may be improved and the coating efliciency increased by theconcurrent selection and control of the degree of the acidity of thesolution and its temperature of application. With regard to thetemperature, it has been found that as the temperaure of the operatingsolution is increased from room temperature, i.e., about 20 degreescentigrade up to about 50 degrees centigrade, the coating rate rapidlyincreases and in some instances it is possible to obtain an increase inthe coating rate of from 2 to 5 times that obtained at room temperature.At solution temperatures within the range of about 50 to about degreescentigrade the coating rate has been found to increase much more slowlyand, for all practical purposes, has been found to be substantiallyuniform throughout this temperature range. It is, therefore, preferredto utilize the solutions of the present invention at solutiontemperatures within the range of about 50 to about 70 degreescentigrade. High temperatures than 70 degrees centigrade may beemployed, for example, temperatures of degrees centigrade or even up tothe solution boiling point, but no particular advantages in terms ofincreased coating rates are obtained by operating at such highertemperatures.

With regard to the pH of the operating solutions of the presentinvention, it has been found that this, as with the temperature, affectsthe coating rate and coating efficiencies of the solution being appliedto the aluminum containing surfaces. Accordingly, it is desirable thatthe coating solution have a pH within the range of about 1.1 to about2.3, and preferably in the range of about 1.6 to about 2.1. This pHrange refers to measurements taken by using an electrical pH meter,employing a glass electrode and a calomel electrode, by immersing theelectrodes in fresh portions of the operating solution and observing theindicated values.

In addition to the pH of the operating solution, it is also desirablethat the operating solution have a concentration within the range ofabout 7 to about 15 points, and that once the concentration isestablished within this range, it is maintained within about 10.5 pointof the established value. The concentration of the operating solution inpoints is determined by the following procedure:

To a ten milliliter sample of the operating solution there is added 25milliliters of 50 percent sulfuric acid and 2 drops oforthophenanthroline ferrous complex (ferroin) indicator. This solutionis then titrated with 0.1 N ferrous sulfate in dilute sulfuric aciduntil the solution changes through blue to a reddish-brown color. Theconcentration points of the operating solution are the number ofmilliliters of the 0.1 N titrating solution used. It is to beappreciated, that although the operating solution of the presentinvention is desirably used at a concentration within the range of about7 to points, operation of the solution at both higher and lower pointconcentrations is not only possible, but in some instances, ispreferred.

Following the application of the coating solution of the presentinvention to the aluminum containing surfaces to be treated, thethus-coated surfaces are then desirably rinsed with water. Either sprayor immersion techniques for the water rinse may be used, with rinsingtimes of about 3 to 5 seconds duration being typical. Following thewater rinse, if desired, the coated surface may be given an additionalrinse with deionized water or with a dilute solution of hexavalentchromium, e.-g., CrO This latter rinse is preferably effected byspraying, rinsing times of about 3 to 5 seconds duration at temperatureswithin the range of about 55 to 65 degrees Centigrade being typical.After the rinsing of the coated aluminum surface has been completed, thesurfaces are preferably dried so as to remove any surface moisture. Thecoatings thus produced on the aluminum surfaces are slightly colored andvary in appearance from iridescent to light gold to yellow to brown. Thecolor changes in the coatings produced may be used as a guide to thecoating weights obtained, the darker colors being produced with highercoating weights and the lighter colors resulting from lower coatingweights.

In formulating the operating solutions of the present invention, amake-up composition, containing the components desired in the operatingsolution, is admixed with water in amounts suitable to provideconcentrations of the components within the ranges as set forthhereinabove. Normally, in addition to the above indicated components,the make-up composition may also contain an inorganic acid such asnitric acid or the like in order to provide the desired acidity or pH.Suitable make-up compositions are those falling within the followingformulation:

Component: Parts by weight CrO 15-20 HF 4-7 Inorganic acid, such as HNO1-5 Molybdenum activator composition 2-5 Al(F) (average A11 3-6 It willbe appreciated that this is a single package makeup composition ascompared to the prior art compositions wherein separate packaging ofsome of the make-up component was often necessary.

In the operation of the process of the present invention, the componentsof the coating solution are depleted. Accordingly, in order to maintainthese components in the operating solution within the preferred rangeswhich have been set forth hereinabove, it is desirable, in order toobtain a continuous operation, to periodically replenish the operatingsolution. One advantage of the present composition is that, as informulating the operating solution, this replenishing may be effectedusing a single package replenishing material, as opposed to many of theprior art compositions wherein separate addition of the componentsduring replenishing is necessary. As with the make-up composition, inaddition to the hexavalent chromium, fluoride, molybdenum and aluminumfluoride complex components, it may also be desirable to include in thereplenishing material an inorganic acid, such as nitric acid, sulfuricacid, hydrochloric acid, and the like, so as to maintain the acidity orpH of the operating solution within the desired ranges as have beenindicated hereinabove. Additionally, where the fluoride component isadded as hydrogen fluoride, rather than fluosilicic acid or fluoboricacid, it may also be desirable to include in the replenishing material aquantity of boric acid, to act as a buffer for the fluoride ions. Thisaddition of boric acid may also be desirable in making up the originaloperating solution, when the fluoride ion is added as hydrogen fluoride,and particularly where the aluminum fluoride complex ion is not includedin the original solution. Where boric acid is included in the originaloperating solution, it is typically added in an amount within a range ofabout 0.1 to about 0.2 percent w./v., although amounts up to its maximumsolubility in the solution may be used.

A single package replenishing material suitable for use in maintainingthe operating solution of the present invention in optimum coatingforming condition may contain the following components in the amountsindicated:

Components: Parts by weight Hexavalent chromium (calculated as CrO 15-20HF 7-12 Acid 1.4-7 Alkali metal molybdate 3-6.5 Al(F) (average AlF0.6-1.6

Additionally, this composition may also contain about 1.2 to about 2parts by weight of boric acid (H BO A preferred replenishing material,having particular utility for use with a continuous strip lineoperation, is one containing the following components in the amountsindicated:

Components: Parts by weight CrO 18-20 HF 10-12 HNO 5-7 Sodium molybdate(Na MOO 2HzO) 3-6 Al(F) (average AlF 0.8-1.5

A preferred replenishing material, particularly suitable for productionoperations in which the parts to be coated are moved through a sprayinstallation on a monorail conveyor, is one having the followingcomponents in the amounts indicated:

Components: Parts by weight CrO 15-17 HF 8-11 HNo 4.5-6.5 Sodiummolybdate (Na MoO .2H O) 3.5-6.5 Al(F) (average AlF 1-1.6

As has been indicated hereinabove, the presence in the operatingsolution of the present invention of foreign cations, that is, cationsother than the sodium, potassium or ammonium ions normally introduced,together with the fluoride or molybdenum ion, have been found to beundesirable and to detrimentally affect the coating rate and coatingefliciencies which may be obtained by using the solution of the presentinvention in the manner in which it has been described above. Inparticular, trivalent chromium ions in amounts in excess of about 0.1percent by weight of the solution have been found to have an appreciablyadverse effect on the coating rate and coating efliciencies obtained.Accordingly, it has been found to be particularly advantageous tomaintain the solutions of the present invention in optimum operatingconditions by operating these solutions in conjunction with an ionexchange unit of the type and by the procedures described in US. Patent2,967,791, issued Jan. 10, 1961. In operating the present solution withsuch an ion exchange unit, which employs a cation exchange resin, thesolutions are maintained free of metallic cations other than aluminum,which occur in the solutions as aluminum floride complex ions, and thesecomplexes are controlled in concentration in the solution by the ionexchange unit.

By the method of the present invention, corrosion resistant adherentcoatings are formed on the aluminum or aluminum alloy surfaces treatedin the matter of a few seconds, typically about 1 to abut 20 seconds.Moreover, by modifying the composition as to the molybdenum content,fluoride content, aluminum fluoride complex content, as well as to thepH and acidity it is possible to maintain a coating rate which issufiiciently fast as to form coatings on continuous strips of aluminumor aluminum alloys, which coatings have a weight in the range of aboutto about 40 milligrams per square foot, in contact times of about 1 to 2seconds.

In order that those skilled in the art may better understand the presentinvention and the manner in which it may be practiced, the followingspecific examples are given. It is to be understood that these examplesare presented for illustrative purposes only and are not intended to betaken as a limitation on the present invention. In these examples,unless otherwise indicated, temperatures are in degrees centigrade andamounts are in percent weight/volume. Additionally, the term CoatingEfficiency refers to the quantity of coating formed relative to theamount of metal dissolved from the surface of the metal being coated andis, specifically, the ratio obtained by dividing the metal loss, inmilligrams per square foot of surface treated, by the coating weight onthe same area, in milligrams per square foot. Thus, as this numericalratio decreases, the coating efliciency increases and the lowest numbersrepresent the highest efliciency of coating formation.

EXAMPLE 1 A solution was prepared containing 0.5 percent CrO 0.3 percentfluoride, as HBF 0.1 percent molybdenum added as Na MoO and 0.37 percentAl(F) (average AlF The solution had a pH of 1.75, a total fluoride of0.55 percent, and after placement in a 2,000 gallon tank was elevated intemperature to 50 degrees centigrade.

Aluminum No. 3105 builder stock was coated by spraying portions of theabove solution thereon to attain contact times with the surface varyingfrom about 7 to about 15 seconds. The resultant coatings were adherent,light-gold in color and had coating weights varying from about to about50 milligrams per square foot, average, depending upon the contact time.The coating efliciency calculated as metal loss in milligrams per squarefoot, divided by the coating weight in milligrams per square footaveraged 0.2. This solution was maintained free of trivalent chromiumions and at the above levelt of aluminum fluoride complex by constantlycycling a small portion of the solution through an ion exchange resincolumn filled with Dowex 50, 20-100 mesh cation exchange resin, andafter many thousands of surface feet of aluminum had been processedthrough the solution, the coating obtained was still similar inappearance and the coating weight and the coating efiiciency remainedsubstantially constant.

EXAMPLE 2 A solution was prepared containing 0.25 percent CrO 0.02percent fluoride, as HF, 0.02 percent molybdenum, added as Na MoO and0.22 percent Al(F) (average AlF The solution had a pH of 1.68, a totalfluoride of 0.17 percent and when applied by spraying at 50 degreescentigrade for 7-15 seconds contact time to the same aluminum builderstock employed in Example 1, produced coatings that were approximatelythe same weight range, had an average coating efiiciency of 0.25.

EXAMPLE 3 A 750 gallon tank for immersion coating was filled with asolution containing 0.5 percent CrO 0.25 percent fluoride as H SlF 0.4percent molybdenum, added as Na MoO 0.23 percent chromic nitrate and0.22 percent Al(F) (average A11 The solution had a pH of 1.55, a totalfluoride of 0.4 percent and was raised in temperature to about 50degrees centigrade. 3003 aluminum processed through the solution byimmersion for seconds to 1 minute produced adherent, gold-coloredcoatings having an average weight of 40.8 milligrams per square foot. Atthis temperature, the average coating efficiency was 0.25.

This solution was operated at varying temperatures by spraying portionsof the solution on 3003 cleaned aluminum panels for a l5-second contacttime. At 38 degrees centigrade the coating weight was 55 milligrams persquare foot, at 43 degrees centigrade, it was 62 milligrams per squarefoot, at 50 degrees centigrade, it was 84 milligrams per square foot, at57 degrees centigrade, it was 81 milligrams per square foot, at 66degrees centigrade, it was 67 milligrams per square foot, and at 71degrees centigrade, it was 50 milligrams per square foot.

EXAMPLE 4 A solution was prepared containing 0.49 percent CrO 0.04percent fluoride, as HF, 0.09 percent molybdenum, added as Na MoO and0.23 percent AMP) (average AlF The solution contained a total fluorideof 0.2 and had a pH of 1.68. The solution was heated to 50 degreescentigrade and sprayed on 3003 aluminum panels for a 15-second contacttime to produce a uniform adherent coating having an average coatingweight of 78 milligrams per square foot at a coating efficiency of 0.19.

The above bath, when operated at 32 degrees centigrade, and underotherwise identical conditions, was found to produce adherent coatinghaving an aver-age weight of 21 milligrams per square foot, whenoperated at 38 degrees centigrade produced coatings having an averageweight of 54 milligrams per square foot; when operated at 54 degreescentigrade produced coatings having an average weight of 98 milligramsper square foot; when operated at 60 degrees centigrade producedcoatings having an average weight of 98 milligrams per square foot; whenoperated at 66 degrees centigrade produced coatings having an averageweight of 86 milligrams per square foot; and when operated at 71 degreescentigrade produced coatings having an average weight of 76 milligramsper square foot.

To portions of the above bath, sodium hydroxide was added sufficient toproduce a pH of 1.81 and under the same conditions of application at 50degrees centigrade and on similar panels, an average coating weight of77 milligrams per square foot was obtained at a coating efficiency of0.20. To another portion of the solution, additional sodium hydroxidewas added to produce a pH of 2.01 and when operated under similarconditions produced coating having an average weight of 71 milligramsper square foot at a coating efiiciency of 0.24. To another portion ofthe above solution, suflicient sodium hydroxide was added to produce apH of 2.12 and when operated under similar conditions was found toproduce adherent coatings having an average weight of milligrams persquare foot at a coating efficiency of 0.27. To another portion of theabove solution, sufficient sodium hydroxide was added to produce a pH of2.28, and when this solution was operated, under similar conditions,adherent coatings were produced having an average weight of 22milligrams per square foot at a coating efiiciency of 0.28.

EXAMPLE 5 Another solution was prepared containing 0.49 percent CrO 0.09percent molybdenum as Na MoO 0.02 percent fluoride, as HF and 0.266percent Al(F) (average AlF This solution had a pH of 1.70 and a totalfluoride content of 0.20 percent. This solution, operating at degreescentigrade was sprayed on 3003 aluminum panels for 15 seconds contacttime and produced an average coating weight of 37 milligrams per squarefoot at a coating efliciency of 0.28. 0.48 percent HNO was added to thissolution to convert the solution into one having a pH of 1.21 and whenoperated under similar conditions produced adherent coatings having anaverage weight of 66 milligrams per square foot at a coating efiiciencyof 0.27.

The above examples are repeated using other sources of hexavalentchromium and fluoride, including sodium and potassium dichromate andfluosilicic acid, respectively, to obtain similar results. The coatingsthus produced are found to be excellent paint base coatings and whenpainted give good results in the percent salt spray, humidity, adhesionand physical tests.

While there have been described various embodiments of the invention,the compositions and methods described are not intended to be understoodas limiting the scope of the invention as it is realized that changestherewithin are possible and it is further intended that each elementrecited in any of the following claims is intended to be understood asreferring to all equivalent elements for accomplishing substantially thesame results in substantially the same or equivalent manner, it beingintended to cover the invention broadly in whatever form its principlesmay be utilized.

What is claimed is:

1. A composition suitable for forming a protective coating onaluminum-containing surfaces which consists essentially of an aqueousacidic solution which contains an eflective coating amount of hexavalentchromium ions, fluoride ions in an amount effect to attack the aluminumsurface being treated and at least 0.01% of molybdenum, as a simplemolybdenum compound.

2. The composition as claimed in claim 1 wherein there is also presentan aluminum fluoride complex.

3. A composition suitable for forming a protective coating on aluminumcontaining surfaces which consists essentially of an aqueous acidicsolution which contains, in percent w./v., 0.05 to 1% hexavalentchromium ions, calculated as CrO 0.16 to 2.7% fluoride ions, and 0.01 to0.4% molybdenum, as a simple molybdenum compound.

4. The composition as claimed in claim 3 wherein the solution alsocontains, in percent w./v., 0.22 to 3.2

" Al(F) complex.

5. The composition as claimed in claim 4 wherein the solution containsin percent w./v., 0.2 to 0.5 hexavalent chromium ions, calculated as CrO0.16 to 1.6 fluoride ions, 0.03 to 0.1 molybdenum, and 0.22 to 1.9 Al(F)complex.

6. A method of forming a protective coating on aluminum-containingsurfaces which comprises contacting the surface to be treated with anaqueous acidic solution which consists essentially of an effectivecoating amount of hexavalent chromium ions, fluoride ions, in an amounteffective to attack the surface being treated and at least 0.01% ofmolybdenum, as a simple molybdenum compound, and maintaining the aqueousacidic solution in contact with the surface to be treated until thedesired protective coating is formed.

7. The method as claimed in claim 6 wherein the aqueous acidic solutionalso contains an aluminum fluoride complex ion.

8. A method for forming a protective coating on aluminum containingsurfaces which comprises contacting the surface to be coated with anaqueous acidic solution which consists essentially of, in percent w./v.,0.05 to 1% hexavalent chromium ions calculated as CrO 0.1 to 2.7%fluoride ions, and 0.01 to 0.4% molybdenum, as a simple molybdenumcompound, and maintaining the aqueous acidic solution in contact withthe surface to be treated until the desired protective coating isformed.

9. The method is as claimed in claim 8 wherein the aqueous acidicsolution also contains, in percent w./v., 0.22 to 3.2 Al(F) complex.

10. The method as claimed in claim 9 wherein the aqueous acidic solutioncontains, in percent w./v., 0.2 to 0.5 hexavalent chromium ions,calculated as CrO 0.16 to 1.6 fluoride ions, 0.03 to 0.1 molybdenum, and0.22 to 1.9 Al(F) complex.

11. A replenishing material for forming solutions for coating aluminumcontaining surfaces which consists essentially of, in parts by weight,15 to 20 CrO 7 to 12 HF; 1.4 to 7 inorganic acid; 3 to 6.5 alkali metalmolybdate; and 0.6 to 1.6 Al(F) (average -AlF 12. The replenishingmaterial as claimed in claim 11 wherein there is contained in parts byweight, 18 to 20 CrO 10 to 12 HP; 5 to 7 HNO 3 to 6 sodium molybdate;and 0.8 to 1.5 Al(F) 13. The replenishing composition as claimed inclaim 11 wherein there is contained in parts by weight, 15 to 17 CrO 8to 11 HF; 4.5 to 6.5 HNO 3.5 to 6.5 sodium molybdate; and 1 to 1.6 Al(F)14. An aluminum surface having a coating thereon produced in accordancewith the method as claimed in claim 6.

15. An aluminum surface having a coating thereon produced in accordancewith the method of claim 10.

16. A make-up composition, suitable for forming aqueous acid solutionsfor coating aluminum containing surfaces which consists essentially of,in parts by weight, 15 to 20 CrO 4 to 7 HF; 1 to 5 inorganic acid; 2 to5 molybdenum activator composition; and 3 to 6 Al(F) (average AlFwherein the activator composition is a water soluble salt of molybdicacid.

References Cited UNITED STATES PATENTS 2,868,679 1/ 1959 Pimbley l486.22,948,643 8/ 1960 Pimbley l486.2 X 2,967,791 I/ 1961 Halversen l486.27 X3,009,842 11/ 1961 Steinbrecher l486.2 3,066,055 11/ 1962 Pimbley l486.2X 3,113,051 12/1963 Pimbley l486.27 X

RALPH S. KENDALL, Primary Examiner.

